A catheter for measuring a flow of blood through a body tissue has a light emitter for emitting light into the body tissue and at least one light receiver for receiving light reflected in the body tissue and back to the catheter. The catheter comprises a first connection tube and a stiff element, wherein the light emitter and the at least one light receiver are arranged in the stiff element. The stiff element comprises at least a first and a second window, wherein the light emitted from the light emitter is emitted through the second window into the body tissue and the light reflected in the body tissue is received by the at least one light receiver through the first window.